material science-b.tech 2012

8/15/2019 Material Science-B.tech 2012

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Material Science


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Objective and scope of the present course

Primary objective is to present the basic fundamentals of materials

science and engineering.

Expose the reader community to different classes of materials, their

properties, structures and imperfections present in them.

Help understand the subject with ease by presenting the content in asimplified and logical sequence at a level appropriate for

students/teachers/researchers.

Aid the teaching learning process through relevant illustrations,

animations, web content and practical examples.

Highlight important concepts for each topic covered in the subject

Provide opportunity of self-evaluation on the understanding of the

subject matter.


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Materials in Day to day life


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What is materials science ?

Material science is the investigation of the relationship among

processing, structure, properties, and performance of materials.


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Introduction

Historical Perspective

Stone → Bronze → Iron → Advanced materials

What is Materials Science and Engineering ?

Processing → Structure → Properties → Performance

Classification of Materials

Metals, Ceramics, Polymers, Semiconductors

Advanced Materials

Electronic materials, superconductors, etc.

Modern Material's Needs, Material of Future

Biodegradable materials, Nanomaterials, “Smart” materials


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Historical Perspective


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A better understanding of structure-composition properties relations has lead to a

remarkable progress in properties of materials. Example is the dramatic progress in

the strength to density ratio of materials, that resulted in a wide variety of new

products, from dental materials to tennis racquets.

M. A. White, Properties of Materials (Oxford University Press, 1999)

Stone age to IT age


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300,000 BC

200,000 BC

Stone age


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5000 BC

5000 BC

4000 BC

3500 BC

Introducing metals


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1450 BC

1500 AD

1855 AD

20th

Century

Iron Age


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The golden era

1886 AD

1890-1910

AD

1939


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The electronic revolution


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throughout the Iron Age many new types of materials

have been introduced

ceramic, semiconductors, polymers, composites…

Age of advanced materials


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Metals: valence electrons are detached from atoms, and spread in an

'electron sea' that "glues" the ions together. Strong, ductile, conduct

electricity and heat well, are shiny if polished.

Semiconductors: the bonding is covalent (electrons are shared between

atoms). Their electrical properties depend strongly on minute proportions

of contaminants. Examples: Si, Ge, GaAs.

Ceramics: atoms behave like either positive or negative ions, and are

bound by Coulomb forces. They are usually combinations of metals or

semiconductors with oxygen, nitrogen or carbon (oxides, nitrides, and

carbides). Hard, brittle, insulators. Examples: glass, porcelain.

Polymers: are bound by covalent forces and also by weak van der Waals

forces, and usually based on C and H. They decompose at moderate

temperatures (100 – 400 C), and are lightweight. Examples: plastics

rubber.

Types of materials


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Materials selection-Properties & cost


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Properties are the way the material responds to the environment

and external forces.

Mechanical properties – response to mechanical forces, strength,etc.

Electrical and magnetic properties - response electrical and

magnetic fields, conductivity, etc.

Thermal properties are related to transmission of heat and heat

capacity.

Optical properties include to absorption, transmission andscattering of light.

Chemical stability in contact with the environment corrosion

resistance.

Properties


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Subatomic level

Electronic structure of individual atoms that

defines interaction among atoms (inter atomicbonding).

• Atomic level

Arrangement of atoms in materials (for the

same atoms can have different properties, e.g.two forms of carbon: graphite and diamond)

• Microscopic structure

Arrangement of small grains of material that can

be identified by microscopy.

• Macroscopic structure

Structural elements that may be viewed with the

naked eye.

Structure


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Aluminum

Does structure changes material?

Glass

Rubber

f l


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Design of materials having specific desired characteristics directly from our

knowledge of atomic structure.

Miniaturization: “Nanostructured" materials, with microstructure that haslength scales between 1 and 100 nm with unusual properties.

Electronic components, materials for quantum computing.

Smart materials: airplane wings that adjust to the air flow conditions,

buildings that stabilize themselves in earthquakes…

Environment-friendly materials: biodegradable or photodegradable plastics,

advances in nuclear waste processing, etc.

Learning from Nature: shells and biological hard tissue can be as strong as the

most advanced laboratory-produced ceramics, mollusces produce biocompatible

adhesives that we do not know how to reproduce…

Materials for lightweight batteries with high storage densities, for turbine

blades that can operate at 2500°C, room-temperature superconductors? chemical

sensors (artificial nose) of extremely high sensitivity, cotton shirts that never

require ironing…

Future of material science


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What is materials science ?

Material science is the investigation of the relationship among

processing, structure, properties, and performance of materials.


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The bonding mechanisms between atoms are closely related to the

structure of the atoms themselves.Atoms = nucleus (protons and neutrons) + electrons

Charges: Electrons and protons have negative and positive charges

of the same magnitude, 1.6 × 10-19Coulombs.

Neutrons are electrically neutral.

Masses: Protons and Neutrons have the same mass, 1.67 × 10-27kg.

Mass of an electron is much smaller, 9.11 × 10-31kg and can be neglected

in calculation of atomic mass.

The atomic mass (A) = mass of protons + mass of neutrons

# protons gives chemical identification of the element # protons =

atomic number (Z) # neutrons defines isotope number

Structure of atoms


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Atomic mass unit (amu) = 1/12 mass of Carbon

12 (12C)

1 mol of substance contains 6.023 x 1023

(Avogadro’s number) atoms or molecules.

Atomic weight = 1 amu/atom (or molecule) = 1

g/mol = Wt. of 6.023 x 1023 atoms or molecules.

For example, atomic weight of copper is 63.54

amu/atom or 63.54 g/mole

Structure of atoms


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•When two neutral atoms are brought close to each other,

they experience attractive and or repulsive force

•Attractive force is due to electrostatic attraction between

electrons of one atom and the nucleus of the other.

Atomic Bonding

Atomic interaction


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Repulsive force arises due to repulsion between electrons

and nuclei of the atoms.

The net force, FN (Fig. a), acting on the atoms is the

summation of attractive and repulsive forces.

The distance, at which the attraction and repulsion forces

are equal and the net force is zero, is the equilibrium

interatomic distance, ro. The atoms have lowest energy at

this position.

Attraction is predominant above ro and repulsion is

dominant below ro (see Fig. a).

Atomic interaction


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Atomic interaction


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is the distance at which the interaction energy is zero.is the depth of the potential well (see Fig. b) and is a

measure of the bonding energy between two atoms.

L-J potential can be also expressed in the simplified form

as VLJ = A/r12 – B/r6 and hence, is also known as 6-12

potential.

A/r12 is predominant at short distances and hence,

represents the short-range repulsive potential due tooverlap of electron orbitals and –B/r6 is dominant at longer

distance and hence, is the long range attractive potential.

Atomic interaction


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The mechanisms of bonding between the atoms are

based on the foregoing discussion on electrostatic interatomicinteraction.

The types of bond and bond strength are determined by

the electronic structures of the atoms involved.

The valence electrons take part in bonding. The atoms

involved acquire, loose or share valence electrons to

achieve the lowest energy or stable configuration of noble

gases.

Atomic bonding can be broadly classified as i) primary

bonding ii) secondary bonding

Atomic Bonding


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Primary bonds

Three types primary bonds are found in solids

Metallic

Ionic

Covalent

Majority of the engineering materials consist of one

of these bonds. Many properties of the materials

depend on the specific kind of bond and the bondenergy

Atomic Bonding

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Ionic bonds are generally found in compounds composed of

metal and non-metal and arise out of electrostatic attractionbetween oppositely charged atoms (ions).

Number of electron in outer shell is 1 in Na and 7 in Cl .

Therefore, Na will tend to reject one electron to get stable

configuration of Ne and Cl will accept one electron to obtainAr configuration. The columbic attraction between Na+ and

Cl¯ions thus formed will make an ionic bond to produce NaCl.

Some other examples are CaF2, CsCl , MgO, Al2O3.

Ionic Bond


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In this type of bonding, atoms share their valence electronsto get a stable configuration.

Methane (CH4): Four hydrogen atoms share their valence

electrons with one carbon atom and the carbon atom in

turn shares one valence electron with each of the four

hydrogen atoms. In the process both H and C atoms get

stable configuration and form a covalent bond.

Covalent Bond


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In metals the valence electrons are not really bound to oneparticular atom, instead they form a sea or cloud of valence

electrons which are shared by all the atoms. The remaining

electrons and the nuclei form what is called the ion core

which is positively charged. The metallic bond arises out of

the columbic attraction between these two oppositely

charged species – the electron cloud and the ion cores.

Metallic Bond


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Ionic and covalent bonds posses high bond energy –450

– 1000 kJ/mole

High bond strength in ionic and covalent solids results inhigh melting point, high strength and hardness. e.g.

diamond

As the electrons are tightly bound to the atoms they are

generally poor conductors of heat and electricity

Are brittle in nature

Characteristics of primary bonds

Structure-property correlation


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Metallic bonds on the other hand provide good thermal and

electrical conductivities as the valence electrons are free tomove.

The metallic bond energy is 68 kJ/mol (Hg) on the lower

side and 850 kJ/mol (W, tungsten) on the higher side.

Bond strength increases with atomic number as more

electrons are available to form the bonds with the ion cores.

As a result melting point, hardness and strength increases

with atomic number.

Metals are ductile as the free moving electrons provides

agility to the bonds and allows plastic deformation.

Structure-property correlation


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Diffusion Phenomena


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Diffusion Mechanism


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Kirkendall Effect


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Kirkendall Effect


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Fick’s lawSteady-state diffusion


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Fick’s second lawNon- Steady-state diffusion

material science-b.tech 2012

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